1. Field of the Invention
The present invention relates generally to an encoder, and more particularly to an encoder in which a coherent beam of light is made incident on a diffraction grating mounted on a movable or rotatable object, a beam diffracted from the diffraction grating being made to interfere with itself to form interference fringes, and the number of light and dark bands included in the interference fringes being counted, thereby measuring the magnitude of travel of the diffraction grating, that is, the magnitude of travel or rotation of the object.
2. Related Background Art
In the fields relating to precision machines such as NC machine tools and semiconductor printing apparatus, a demand has recently arisen with respect to precision measuring instruments capable of making measurements in a unit of 1 .mu.m or less (submicron).
As a typical measuring instrument capable of making measurement in a unit of submicron, a linear encoder employing interference fringes has heretofore been known in which a coherent beam of light such as a laser beam is used to obtain a beam diffracted from a moving object, thereby forming the interference fringes.
The aforementioned type of linear encoder is disclosed, for example, in U.S. Pat. Nos. 3,738,753, 3,726,595 and 4,676,645; Japanese Utility Model Laid-open No. 81510/1982; and Japanese Patent Laid-open Nos. 207805/1982 and 19202/1982.
FIG. 1 is a schematic illustration of the construction of one example of a prior-art linear encoder. As shown in FIG. 1, the linear encoder includes a laser 1, a collimator lens 2 and a diffraction grating 3 of a grating pitch d mounted on a movable object (not shown), the diffraction grating 3 being moved, for example, at a velocity v in the directions indicated by a doubleheaded arrow shown.
The linear encoder also includes quarter-wave plates 4.sub.1, 4.sub.2, roof prisms or corner cube reflection mirrors 5.sub.1, 5.sub.2 for preventing the optical axis of a re-diffracted beam from being shifted by the inclination of the diffraction grating 3, a beam splitter 6, polarizing plate 7.sub.1, 7.sub.2 in which their axes of polarization are cross perpendicular to each other and are arranged to form an angle of 45.degree. with respect to the respective polarization axes of the quarter-wave plates 4.sub.1, 4.sub.2, and light receiving element 8.sub.1, 8.sub.2.
Referring to FIG. 1, the laser beam emitted from the laser 1 is collimated into a substantially parallel beam by the collimator lens 2, then being made incident on the diffraction grating 3. Positive and negative lights diffracted into "positive and negative m" orders by the diffraction grating 3 are respectively passed through the quarter-wave plates 4.sub.1, 4.sub.2, then reflected by the corner cube reflection mirrors 5.sub.1, 5.sub.2. The respective reflected beams are again made incident on the diffraction grating 3, then re-diffracted into "positive and negative m" orders, and superposed on each other. The beam superposed is split into two beams of light by the beam splitter 6, and the beams are respectively made incident on the light receiving elements 8.sub.1, 8.sub.2 through the polarizing plates 7.sub.1, 7.sub.2.
The beams incident on the light receiving elements 8.sub.1, 8.sub.2 are 90.degree. out of phase with respect to each other through a combination of the quarter-wave plates 4.sub.1, 4.sub.2 and the polarizing plates 7.sub.1, 7.sub.2, such incident beams being used for discrimination of the direction of travel of the diffraction grating 3. Thus, the magnitude of travel of the diffraction grating 3 is calculated by counting the number of light and dark bands of the interference fringes received by the light receiving elements 8.sub.1, 8.sub.2.
FIG. 2 is a schematic illustration of another example of a prior-art linear encoder employing a diffracted beam which is transmitted. As shown in FIG. 2, in order to reduce the overall width of the system, a reflection prism 9 is used to bend the beam emitted from the laser 1 and the beam diffracted from the diffraction grating 3, a transmitted diffracted beam being utilized as a diffracted beam. The other arrangement is the same as that of the linear encoder shown in FIG. 1.
In the respective linear encoders shown in FIGS. 1 and 2, the light beam is again made incident on the diffraction grating 3 by means of reflection means such as roof prisms and corner cube reflection mirrors.
With this arrangement, even if the wavelength of the laser 1 is varied, for example, by factors such as ambient temperature and the angle of diffraction of the diffraction grating 3 is changed, the diffraction grating 3 is again illuminated by the respective beams consistently at the same angle, and thus the two rediffracted beams are necessarily superposed on each other, thereby properly maintaining the S/N ratios of the signals output from the light receiving elements 8.sub.1, 8.sub.2.
However, when the roof prisms and the corner cube reflection mirrors are to be disposed, they need to be placed at locations at which they do not intercept a zero diffracted order beam. For example, where the grating pitch of the diffraction grating 3 is 3.2 .mu.m and the wavelength of the beam used by the laser 1 is 0.83 .mu.m, if a first diffracted order beam is employed, the angle of diffraction is sin.sup.-1 (0.83/32)=15 degrees. In order to separate the zero diffracted order beam and the reflection means, if the reflection means is disposed at a position, for example, 15 mm away from the normal to the diffraction grating 31 (the direction of the optical axis of the zero diffracted order beam) at a location where the beam is incident on the diffraction grating 3, the reflection means must be disposed at a location which is remote from the diffraction grating 3 by 15/ tan 15.degree.=56 (mm). Therefore, the use of the roof prisms and the corner cube reflection mirrors cannot avoid an increase in the overall size of the system.
Rotary encoders of interference fringe detection types have previously been disclosed in U.S. Ser. No. 07/393,104 and U.S. Pat. Nos. 4,829,342 and 4,868,385 which are assigned to the same assignee as in the case of the present application. In these types of rotary encoders, if roof prisms and corner cubes are employed in order to again make diffracted a beam incident on a diffraction grating, this is a large obstacle to a reduction in the overall size of the system.
In general, roof prisms and corner cubes involve disadvantages in that highly accurate working is necessary, production thus being difficult, and a high cost thus results.